This paper describes the effects of system LSI design with C language-based behavioral synthesis following several trials of design period reduction and quality improvement for a variety of circuit types. The results of these trials are analyzed from the viewpoints of description productivity, verification productivity, reusability and design flexibility as well as hardware and software co-verification. First the C-based design flow proposed by the authors is described, and the design productivity and verification productivity under this design flow is compared to RTL design. The reusability of the behavioral IP core and its efficiency with HW/SW co-verification are also shown using design examples. Next, using the example of an MPEG-4 video decoder design, a typical design process in a C-based design is shown with considerations regarding verification efficiency, reusability of the IP core and HW/SW co-verification. Finally, the authors' perspectives regarding future directions of system LSI design are discussed.